The present invention relates to a brushless motor, and more particularly, to a brushless motor including an improved bus bar and an electric power steering apparatus using such a motor.
An electric power steering (EPS) apparatus using a motor as a drive source is nowadays often used as a vehicle power steering device. Such an EPS uses a brushless motor including a stator core, which has teeth extending radially inward, and a coil, which is formed by winding a conductive wire onto each tooth. Japanese Laid-Open Patent Publication No. 2006-296147 describes a prior art example of a brushless motor. In such a motor, an independent conductive wire is wound on each tooth to form a coil. The motor includes a bus bar for connecting ends of the coil supplied with current of the same phase and for supplying each coil with current from an external device.
In the above-described motor, the bus bar is arranged in a housing in a state embedded in a bus bar holder formed from a resin. Resin usually has low thermal conductivity. Thus, when current flows though a coil and generates heat, the bus holder that is arranged in the housing makes it difficult to radiate the heat out of the housing. Furthermore, an increase in the temperature of the coil increases the electrical resistance of the coil. This increases voltage drop at the coil, which in turn may lower output of the motor. In such a case, the EPS cannot apply an appropriate assist force to a steering system when sudden steering (high-speed steering) operations are performed. This may result in an uncomfortable steering feel.